New York City Police Commissioner Bill Bratton talks with onlookers after leaving flowers at an impromptu memorial near the site where two police officers were killed in the Brooklyn borough of New York, Sunday, Dec. 21, 2014.

Moments before shooting dead two police officers in Brooklyn on Saturday, Ismaaiyl Brinsley told two passers-by to follow him on Instagram and said "watch what I am going to do."

Brinsley also took a Bolt bus to New York from the Baltimore area on the morning of the attack, arriving on Manhattan's west side around 10:15 a.m.

The new details of the attack emerged during a NYPD press briefing on Sunday afternoon where investigator began to paint a picture of the hours before the attack. NYPD Chief of Detectives Robert Boyce said that while Baltimore Police had faxed a wanted post of Brinsley to the department, it came in just one minute before the attack. According to Boyce, the fax arrived at 2:46 p.m. EST and "a minute later is the homicide."

During the briefing, police also shared an image of Brinsley and the gun that he used in the attack, which someone (not Bringsley) purchased at a pawn shop in Georgia in 1996. It is unclear how he got the gun.

Chief Boyce said that prior to the attack, Brinsley had an interaction with two witnesses on the street. He asked them for their gang affiliation before he told them to follow him on Instagram.

Before the attack, Brinsley posted two anti-police posts on his Instagram account. The account has since been disabled, but a screenshot of the messages can be seen below, one of which says "I am putting wings on pigs today."

Wow, this is supposedly from the Brooklyn gunman's Instagram who shot the two police officers in Brooklyn. pic.twitter.com/9PWdHW7Enm

The gunman was chased by two power company workers who reported the shooting to police nearby.

During the attack, Brinsley fired four shots at the officers Wenjian Liu and Rafael Ramos and later shot himself one time. Both officers died in the hospital.

It is believed that Brinsley suffered from undiagnosed mental health issues and did take medication at some point, according to Boyce. The information was provided by family members, who said Brinsley had attempted to hang himself in the past, and was estranged from two sisters.

About two hours and 40 minutes of time that Brinsley spent in New York on Saturday were not accounted for, according to Boyce, who asked anyone with information to contact the NYPD.